Production of cellulose triesters



April 14, 1953 J. P. STONEMAN 2,635,097

PRODUCTION OF CELLULOSE TRIESTERS Filed April 12, 1951 l2 3 8 f\ I PX |o //7 venfor Ja/zn Ph/Yp Sfoneman By his afforneys Patented Apr. 14, 1953 PRODUCTION OF CELLULOSE TRIESTERS John P. Stoneman, Coventry, England, assignor' to Courtaulds Limited, London, England, a

British company Application April 12, 1951, Serial No. 220,592 In Great Britain May 12, 1950 2 Claims. (01. 260-227) 1 This invention relates to the production of cellulose triesters.

It is known, see for example United States patent specification No. 1,861,320, that cellulose in fibrous form may be esterified whilstmaintaining its fibrous structure by treating the fibres with an esteriiying solution comprising a fatty acid anhydride, an esterification catalyst and an inert solvent, that is to say a solvent which will dissolve the anhydride and the catalyst but which will not dissolve cellulose or the cellulose triester.

It has also been proposed in British Gevaert Photo-Producten, N. V. patent specification No. 19,210/46 as laid open to public inspection and its corresponding French patent specification No. 929,968, to produce cellulose esters in fibrous form by treating a stationary cellulose mass with an esterifying liquid which is passed through the mass of cellulose. Thisprocess is difiicult to carry out economically on a'commercial scale as the resistance to flow of the liquid increases during the conversion of the cellulose into the triester; the resulting falling off of the rate of flow leads to irregular heating in the cellulosic material and the production of non-uniform triesters of little value.

The object of the present invention is to provide an eflicient and economic process for the production of cellulose triesters in fibrous form.

According to the present invention, in a process for the manufacture of cellulose triesters by passing an esterifying liquid comprising a fatty acid anhydride, an esterification catalyst and an inert solvent through a static bed of cellulose under such conditions that the fibrous form of the cellulosic material is maintained, the direction of the flow of the esterifying liquid is reversed at least once during the esterification in order to maintain the required flow of liquid through the bed of cellulosic material.

In the early stages of the esterification when the cellulose is being converted into a cellulose triester, the solid phase swells with the result that it tends to offer an additional resistance to the flow of the esterifying liquid. The result is that with a unidirectional flow a compressed pad is usually formed near the top or bottom of the bed of cellulosic material depending on whether an upward or downward flow, respec-- tively, is used. In a preferred form of the in vention the esterification liquid is passed one way through the bed of cellulosic material until a marked resistance to its flow is developed and at this stage the flow is reversed so that the re- 2 sistance of the bed to the flow is substantially reduced. With certain forms of cellulose, once the flow has been reversed as described the resistance to flow remains sufficiently low to enable the esterification to be completed with this single reversal. With other forms of cellulose however, the resistance to flow may gradually build up again to such an extent that it is desirable to reverse the flow'more than once before the esterification is completed.

One suitable form of apparatus for carrying out the present invention is diagrammatically pump 2 through pipe 6.

illustrated in the accompanying drawing. The cellulose is packed in a reaction'vessel l connected directly to a pump 2 and a reservoir 3 by pipes 4 and 5 respectively while the pump 2 and reservoir 3 are connected together by a pipe 6. The pipes 4 and 5, which contain valves 1 and 8 respectively, are connected by two other pipes 9, l0 which contain valves II, 12 respectively. To pass the esterification liquor upwardly through the bed of cellulose the valves 1 and 8 are open and the valves Hand I2 are closed and the esterification liquor passes from pump 2 through the pipe 4, through the vessel I, through pipe 5 into thereservoir 3 and back to the pump 2 through pipe 6. To reverse the flow, valves 1 and 8 are closed and valves H and 12 are opened; the liquor then passes from the pump 2 through the pipe 4 to the byepass with pipe 9, through pipe '9 into pipe 5 above the valve 8, from pipe 5 into and through the Vessel I, through pipe Ill into pipe 5 below the valve 8, into the reservoir 3 and back to the In another form of the invention the static bed of cellulosic material is arranged in a reaction vessel which is connected to a reservoir for the esterifying liquid, a heat exchanger, a pump and a reversing manifold, the apparatus forming a closed liquid circulating system.

Esterification of cellulose is an exothermic reaction and it is desirable that the rate of flow of the esterification liquid and the reversal or reversals of the flow should be so arranged that overheating of the cellulosic material in the reaction vessel is avoided and the optimum temperature of reaction maintained.

The cellulose employed in the present invention may be in the form of cotton linters, wood pulp or mixtures of the two. The cellulose may also be pretreated, for example with water or with the appropriate fatty acid, before being esterified in accordance with the invention.

Suitable catalysts for use in the invention are sulphuric acid, perchloric acid and mixtures of these two acids.

The invention is particularly applicable to the production of cellulose triacetate using an esterifying liquid containing acetic anhydride. Carbon tetrachloride is a suitable inert solvent for use in acetylation.

The invention is illustrated by the following examples in which parts and percentages are by weight.

Example "1' per cent perchloric acid and, 0.022 part of sulphuric acid was pumped upwards through the bed of cellulose ata linear velocity of 1 foot per minute. The liquor was initially at 15 centigrade; after 15 minutes its temperature had risen to 25 centigrade and the. pressure d'rop'across th bed was 14 pounds per square inch. The flow was then reversed whereupon the pressure drop across the bed immediately fell to 3 pounds per square inch and it remained at between 3 and '7 pounds per square inch until acetylation was complete. After 35 minutes, the temperature had risen to 35 centigrade and the mixture was kept at that temperature for another 105 minutes to complete the acetylation. Suillcient sodium acetate was added to neutralise the catalyst, the acetylation liquor was drained off and the product was washed several times with carbon tetrachloride to remove substantially all the remaining acetic acid and anhydrid'e. The carbon tetrachloride was then removed and the product, after being washed with water until it. was free from acid, was dried.

Example 2 1 part of cellulose (purified wood pulp) was pretreated by spraying it with 0.4 part of acetic acid whilst it was agitated by means of rotating blades in a 'closed vessel. After agitating for 4 hours at 25 centigrade the pretreated cellulose wa transferred to a reaction vessel to form a bed 3 feet deep. An acetylation liquor, initially at 15 centigrade, consisting of a mixture of 4.5 parts of 95 per cent acetic anhydride, 34.0 parts of carbon tetrachloride and 0.011 part of 60 per .4 per cent perchloric acid was pumped through at a linear velocity of 1 foot per minute. After 5 minutes the temperature of the liquor had risen to 20 centigrade and the pressure drop across the bed had risen to 15 pounds per square inch. The flow was reversed, the pressure drop then falling to 5 pounds per square inch. As'acetylation proceeded the pressure drop slowly built up again to 14-15 pounds per square inch, and the how was reversed again after 12, 20, 32 and minutes respectively. The temperature of the acetylationliquor after 60 minutes had reached 30 centigrade and heat was applied to raise its temperature to 35 Centigrade. The acetylation was thencontinued at that temperature until the total acetylation time was minutes. Further reversals of flow were not necessary, as a flow rate'of 1 foot per minute was then maintained. The cellulose triacetate was separated as described in Example 1.

What I claim is:

1. A process for the manufacture of cellulose triacetate bypassing an, acetylation liquid comprising acetic anhydride, an esterification catalyst and carbon tetrachloride through a static bed of cellulose in fibrous form wherein the fibrous form of the cellulosic material is maintained, until the cellulose is substantially converted into cellulose triacetate, and the direction of the flow of the acetylation liquid is reversed each time the pressure drop across the bed rises to at least 14 pounds per square inch, whereby the flow of the liquid through the bed is maintained throughout the acetylation.

2. A process forth'e manufacture of cellulose triacetate by passing an esterifying liquid comprising acetic anhydride, an esterification catalyst and an inert solvent through a static bed of cellulose in fibrous form whereinthe fibrous form of the cellulosic material is maintained until the cellulose is substantially converted into cellulose triacetate and the direction of the flow of the esterification liquid is reversed each time the pressure drop across the bed rises 'to at least 14 pounds per square inch, whereby the flow of the liquid through the bed is maintained throughout the esterification.

J. P. STONEMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,861,320 Rheiner May 31, 1932 2,076,181 Hopkinson etal. Apr. -6, 1937 2,084,833 Berl a- June 22, 1937 

1. A PROCESS FOR THE MANUFACTURE OF CELLULOSE TRIACETATE BY PASSING AN ACETYLATION LIQUID COMPRISING ACETIC ANHYDRIDE, AN ESTERFICATION CATALYST AND CARBON TETRACHLORIDE THROUGH A STATIC BED OF CELLULOSE IN FIBROUS FORM WHEREIN THE FIBROUS FORM OF THE CELLULOSIC MATERIAL IS MAINTAINED, UNTIL THE CELLULOSE IS SUBSTANTIALLY CONVERTED INTO CELLULOSE TRIACETATE, AND THE DIRECTION OF THE FLOW OF THE ACETYLATION LIQUID IS REVERSED EACH TIME THE PRESSURE DROP ACROSS THE BED RISES TO AT LEAST 14 POUNDS PER SQUARE INCH, WHERBY THE FLOW OF THE LIQUID THROUGH THE BED IS MAINTAINED THROUGHOUT THE ACETYLATION. 